It is known for the transmission of an agricultural vehicle, such as a tractor or a harvester, to incorporate a CVT acting in conjunction with a change speed gearbox offering multiple drive ratios. One known form of CVT is a hydrostatic transmission which comprises a variable displacement pump connected by hydraulic lines to a hydraulic motor. The torque applied to the wheels is set in this case by varying the displacement of the pump. The ensuing description will be made by reference to such a hydrostatic transmission but it should be mentioned that other forms of continuously variable transmission, such as a belt, chain, or electrically driven CVT can be used in place of a hydrostatic transmission, and reference to changing the displacement of the pump should be construed as being equivalent to varying the transmission ratio of the CVT by other means, such as by varying the geometry of the cone sheaves of a belt driven CVT.
To set the torque applied to the drive wheels by the engine, a controller is provided which varies the displacement of the pump in dependence upon the difference between the desired and actual values of the wheel speed. Thus, if the vehicle speed or the engine speed drops below the desired value, the pump displacement is adapted to match the desired vehicle or engine speed.
It is sometimes necessary to apply drive torque to the wheels at the same time as the brakes are applied. For example, at the commencement of a hill start, the engine may provide torque to counteract the force of gravity at the same time as the brakes are applied to hold the vehicle still. When the brakes are then released, the vehicle moves forward gradually. Also when driving downhill, the engine may be used in conjunction with the brakes to retard the vehicle.
If the control system setting the transmission ratio of the CVT takes no account of the applied braking torque, it cannot distinguish between applied braking torque and the reaction torque of the vehicle wheels when standing on an incline. The CVT transmission ratio will therefore be varied in order to counteract the effect of the brakes.
For this reason, it has previously been proposed to detect when the brakes are applied and, under such conditions, the transmission ratio is open loop controlled. This is not entirely satisfactory as it leads to unnecessary brake wear, it makes it difficult to achieve smooth hill starts and it risks overheating of the brakes during prolonged driving downhill.